Due to FCC power limitations, geographical and other factors, it is sometimes not possible for a single RF transmitting site to provide adequate coverage to a large desired coverage area. For example, government entities commonly use land-mobile radio communications systems to provide communications between a headquarters and various mobile and portable radio users that rove throughout the jurisdiction of the governmental entity. In some cases the geographical area of jurisdiction is so large that it is not possible for a single land-based RF transmitting site to cover it. Even if the effective radiated power of the single transmission site was sufficiently great to cover the entire area, users in outlying or fringe areas might receive only spotty service because of the "line-of-site" nature of VHF transmissions and/or due to geographical obstructions (e.g., hills, bridges, buildings, and the curvature of the earth) interposed between the single transmitter site and various fringe locations within the coverage area.
One known way to expand the coverage area is to provide multiple, "simulcasting" transmitting sites. In order to simplify mobile radio operation and conserve radio frequency spectrum, such "simulcasting" RF transmitting sites all transmit substantially identical signals at substantially identical times on substantially identical radio frequencies. Such "simulcasting" eliminates control overhead and other complexities associated with performing "hand offs" from one RF transmitting site coverage area to another as is common, for example, in cellular and "multi-site" RF communications system. So-called "simulcasting" digitally trunked RF repeater systems are generally known. The following is a listing (which is by no means exhaustive) of prior documents that describe various aspects of RF transmission simulcasting and related issues:
U.S. Pat. No. 4,696,052 to Breeden; PA1 U.S. Pat. No. 4,696,051 to Breeden; PA1 U.S. Pat. No. 4,570,265 to Thro; PA1 U.S. Pat. No. 4,516,269 to Krinock; PA1 U.S. Pat. No. 4,475,246 to Batlivala et al.; PA1 U.S. Pat. No. 4,317,220 to Martin; PA1 U.S. Pat. No. 4,972,410 to Cohen et al.; PA1 U.S. Pat. No. 4,903,321 to Hall et al.; PA1 U.S. Pat. No. 4,608,699 to Batlivala et al.; PA1 U.S. Pat. No. 4,918,437 to Jasinski et al.; PA1 U.S. Pat. No. 4,578,815 to Persinotti; PA1 U.S. Pat. No. 5,003,617 to Epsom et al.; PA1 U.S. Pat. No. 4,939,746 to Childress; PA1 U.S. Pat. No. 4,903,262 to Dissosway et al.; PA1 U.S. Pat. No. 4,926,496 to Cole et al.; PA1 U.S. Pat. No. 4,968,966 to Jasinski et al; PA1 U.S. Pat. No. 3,902,161 to Kiowaski et al; PA1 U.S. Pat. No. 4,218,654 to Ogawa et al; PA1 U.S. Pat. No. 4,255,815 to Osborn; PA1 U.S. Pat. No. 4,411,007 to Rodman et al; PA1 U.S. Pat. No. 4,414,661 to Karlstrom; PA1 U.S. Pat. No. 4,472,802 to Pin et al.; PA1 U.S. Pat. No. 4,597,105 to Freeburg; and
Japanese Patent Disclosure No. 61-107826.
While simulcasting thus provides various advantages as compared to other techniques for expanding coverage area, it also introduces its own particular set of complexities that must be dealt with. By way of illustration, please refer to FIG. 1--which is a schematic diagram of an exemplary three-site simulcasting digitally trunked land-mobile RF communications system 10. System 10 includes three simulcasting transmitting sites, S1, S2 and S3. The transmissions of site S1 cover the coverage area A1, and similarly, the transmissions of sites S2 and S3 cover respective coverage areas A2, A3. A central control point C coupled to each of sites S1, S2 and S3 via a respective communications link (L1-L3) delivers, in real time, substantially identical signalling (including digital control channel signalling and associated timing information) for transmission by the various sites.
Exemplary system 10 is preferably a digitally trunked communications system of the type marketed by Ericsson-GE Mobile Communications Inc. under the trade name EDACS. This system provides a digital RF control channel and plural RF working channels. In such a digitally trunked system, an exemplary mobile radio unit M within one (or more) of coverage areas A1-A3 continuously monitors the "outbound" digital control channel when it is not actually engaged in active communications on a working channel with other units. Mobile M may request communications by transmitting a channel assignment request message on the "inbound" control channel. Upon receipt of such channel assignment request (and presuming that at least one working channel is available for temporary assignment to mobile unit M and the other units that mobile M wishes to communicate with), control point C responds by causing a trunking control channel assignment message to be transmitted by each site S1-S3 over the outbound control channel. In simulcast system 10, this channel assignment message is transmitted simultaneously by each of transmitting sites S1-S3 over the same outbound control channel frequency (such that mobile unit M and other mobile units "called" by the channel assignment message will receive the message regardless of which of coverage areas A1-A3 they may happen to be located within). Mobile unit M (and other called mobile units) respond to the received outbound trunking control channel assignment message by changing frequency to an RF working channel and conducting communications on the working channel. Once the working channel communications are concluded, the mobile unit M (and other called mobile units) return to monitoring the outbound control channel for additional messages directed to them.
Commonly assigned U.S. Pat. Nos. 4,905,302 and 4,939,746 provide additional detail regarding the exemplary trunking control process described above and also describe in detail the signals which are transmitted over the outbound control channel. Briefly, the outbound control channel signalling is "slotted" or "framed" with the different message slots being defined by synchronization signalling which is periodically transmitted over the outbound control channel. In preferred system 10, a dotting/Barker code sequence used for synchronization purposes recurs on the outbound control channel every 30 milliseconds. Message slots are defined between such recurring dotting/Barker synchronization signal transmissions. The timing of such dotting/Barker transmissions is set at control point C by a master time base frame synchronization link (FSL) signal. Control point C embeds such timing information into control channel signalling it sends to each of sites S1-S3 via respective links L1-L3.
Referring once again to FIG. 1, suppose mobile unit M is located within an overlap area X wherein coverage areas A2 and A3 overlap one another. Within this overlap area X, mobile unit M will receive (perhaps at approximately equal signal strength levels) the outbound control channel transmission of site S2 and also the outbound control channel transmission of site S3. Simulcast system 10 is appropriately designed such that such outbound control channel transmissions from sites S2 and S3 are on substantially the same RF frequency so that no heterodyning or other interference occurs. Similarly, control point C sends, over links L1-L3, substantially identical outbound control channel messages for transmission by each of sites S1-S3.
However, a problem can arise if the outbound control channels are not precisely synchronized to one another. A transceiver located within overlap region X that receives outbound control channel synchronization signals delayed with respect to one another by even a small time period (e.g., more than about 1-half bit period, or 52 microseconds for 9600 baud operation) could end up losing bits and/or temporarily losing synchronization, bit recovery and error checking capabilities.
Delays due to the limited speed at which electromagnetic waves propagate must be taken into account in systems simulcasting data at high data transmission rates (an RF signal travels "only" about 300 meters in one microsecond). It is possible (and usually necessary) to adjust the relative effective radiated power levels of the site transmitters so that the distances across the overlap regions X are kept less than a desired maximum distance--and thus, the difference in the RF propagation delay times across an overlap region due to the different RF path lengths between the site and a receiver within the overlap region is minimized. Even with this optimization, however, it has been found that (due to the additional differential delay caused by the different RF path lengths) a maximum system differential delay stability of .+-.5 microseconds must be observed to guarantee that the transceiver in any arbitrary location within a typical overlap region X will receive the corresponding digital signal bit edges within 52 microseconds of one another.
Fortunately, it is typically possible to minimize time delay differences to on the order of a microsecond through various known techniques. For example, it is well known in the art to introduce adjustable delay networks (and phase equalization networks) in line with some or all of links L1-L3 to compensate for inherent differential link delay times (see U.S. Pat. No. 4,516,269 to Krinock, and U.S. Pat. Nos. 4,696,051 and 4,696,052 to Breeden, for example). Typical conventional microwave link channels exhibit amplitude, phase and delay characteristics that are extremely stable over long periods of time (e.g., many months), so that such additional delays, once adjusted, guarantee that a common signal input into all of the links L1-L3 at the same time will arrive at the other ends of the links at almost exactly the same time. The same or additional delays can be used to compensate for different, constant delay times introduced by signal processing equipment at the sites S1-S3 to provide simultaneous coherent transmission of the signals by the different sites. For example, the above-identified Rose et al. patent application describes a technique wherein additional frequency and timing information is provided to each site over one or more additional channels in order to eliminate timing ambiguities that may result from the use of conventional multi-level, multi-phase protocol-type modems.
Even in well-designed simulcasting systems, however, various abnormal factors (e.g., electromagnetic noise and spikes resulting from lightning strikes and the like) can cause a properly operating simulcasting system to lose synchronization. A "hit" or outage effecting a particular data path and its associated modems may cause the timing to be reestablished at a "random" latency. Since the timing and location of these "hits" is not predictable and, moreover, may occur remotely from the control point C, it may be difficult to detect the timing fault, relay this information back to the control point, and initiate effective action.
In 1989, the assignee installed a control channel and working channel resynchronization arrangement in a customer's simulcast system. Such resynchronization arrangement acted to periodically reestablish data timing on the control channel and on working channels. In this simulcast system, the control channel ("CC") formerly carried a continuous data stream that did not provide "gaps" at every call which might be used for resynchronization purposes. The data stream did provide gaps periodically to provide a short time period (e.g., 11 milliseconds long) of all "1's" to periodically cause a resynch. This period is sufficiently brief and appropriately located so that the data framing is left intact; and is chosen to be placed in the data stream every certain time period (e.g., every 54 seconds).
Thus, it is known to resynchronize the control channel periodically (e.g., every 54 seconds) on a routine basis in order to correct any control channel timing errors that may arise in simulcast system 10. In addition, the above-identified U.S. patent application Ser. No. 07/824,123 filed Jan. 22, 1992 in the name of the present applicant describes additional techniques (which have been in public use for more than a year and are therefore prior art to the present application) for periodically "kicking" a modem in order to ensure that the modem uses a distributed common clocking signal; and for retraining a communications link and associated modems for a simulcast system working channel if a routinely performed working channel "test call" fails.
Thus, actual "over-the-air" monitoring has been successfully used in the prior art to periodically test working channel timing using the "test call" technique. Unfortunately, the prior art "test call" approach described in copending application Ser. No. 07/824,123 cannot be used for testing control channel synchronization because the control channel is always in use and cannot conveniently be temporarily taken out of service for testing--and because such "test calls" in exemplary system 10 make use of a local test transceiver that is itself synchronized with the locally transmitted outbound control channel and has no other timing reference. See U.S. Pat. No. 4,903,321 to Hall et al. Thus, although such an exemplary "test call" transceiver can test working channel timing relative to the control channel, it is incapable of comparing control channel timing to any other timing reference. While periodic resynchronization of the control channel every minute or so as described above will (absent some failure more fundamental non-momentary timing malfunction) successfully resynchronize control channel timing of all simulcast sites S1-S3 relative to one another, it has not in the past been possible (due to the fact that the control channel continuously carries messaging traffic) to monitor over-the-air control channel signalling to provide such resynchronization when actually needed. Loss of relative timing synchronization between the control channels of two or more simulcast sites S1-S3 for even a few seconds may cause mobile units M in overlap areas to temporarily lose contact--a condition which is annoying and may also lead to missed calls. Thus, a still more reliable mechanism for continuously ensuring control channel timing synchronization within a digitally trunked simulcast RF communications repeater system would be highly desirable.
The present invention provides highly reliable continuous common point control channel timing detection and correction within a simulcast system. In accordance with one aspect provided by the present invention, a control channel timing monitor receives control channel timing signals transmitted over-the-air on the outbound control channel by each of the simulcasting transmitting sites. The control channel monitor extracts timing information from each of the monitored signals, and analyzes such timing information to determine relative and/or absolute synchronization.
In one preferred embodiment, the extracted timing information is compared to a system-wide master timing reference (this master timing reference available at the control point is used to synchronize all signals provided to simulcasting transmission sites S1-S3). The monitor provides a delay of reference timing relative to the received monitored control channel signalling in order to compensate for inherent system delays. An alarm is generated if the comparison indicates that control channel timing of any of the simulcasting transmission sites is not synchronized with the master timing reference. Corrective action may be immediately taken in response to the alarm.
In accordance with a further aspect provided by the present invention, corrective action may include sending a command to the simulcast control site that causes system 50 to take the faulty control channel out of service as a control channel and substitute a working channel frequency and associated equipment for the control channel equipment. Such shifting of the control channel to a different frequency is accomplished very rapidly in preferred system 50 in accordance with known techniques described, for example, in commonly assigned patent application Ser. No. 07/532,164 filed Jun. 5, 1990 entitled FAIL-SOFT ARCHITECTURE FOR PUBLIC TRUNKING SYSTEM (attorney reference number 46-72: Client reference number 45-MR-541). In accordance with this aspect provided by the present invention, the hardware formerly operating on the control channel is left in service as a working channel, and a "test call" may be directed to that working channel (as described in application Ser. No. 07/824,123). Such "test call" will, if it fails, cause retraining of the associated communication link and modems as described in application Ser. No. 07/824,123 (thereby possibly resolving the loss of synchronization problem)--or the channel will be taken out of service entirely if a more severe failure mode is present.